HIGH-DENSITY GAS IN THE CORE OF THE SAGITTARIUS-B2 MOLECULAR CLOUD

We have observed several transitions of HC3N in the ground and vibrationally excited states in order to determine the density and kinetic temperature in the core of the Sagittarius B2 molecular cloud. Statistical equilibrium analysis of the J = 12 --> 11 and 4 --> 3 data suggests that warm and dense gas with a kinetic temperature of approximately 20-40 K and a mean H2 density of approximately 10(5) cm-3 is present in the inner region of the cloud approximately 10 pc in size. The HC3N column density does not correlate well with the C18O column density, suggesting strong density, suggesting strong variations of the HC3N fractional abundance in the region by a factor of approximately 3 on a 1' scale. The HC3N fractional abundance derived at the center of the cloud is approximately 3 x 10(-10), comparable to that derived for the ridge component of the Orion KL nebula. Detection of the J = 39 --> 38 transition of HC3N suggests H2 densities greater than approximately 10(7) cm-3 and mean kinetic temperatures greater than approximately 60 K and 80 K in 21" FWHM beams centered at the positions of the Sgr B2(M) and (N) sources. Because this transition is most likely collisionally excited, the higher intensity and excitation temperature derived at the position of the Sgr B2(N) continuum source indicate higher density in the northern core compared to the middle core. Vibrationally excited HC3N gives an approximately 260 K estimate for the mean radiation field temperature for both cores in a 45" FWHM beam.